In recent years, a technique to manufacture a thin film transistor (hereinafter referred to as a TFT) over a substrate has made great progress and application to an active matrix display device has been advanced. In particular, since a TFT using a poly-crystalline semiconductor film has higher field-effect mobility than a conventional TFT using an amorphous semiconductor film, high-speed operation has become possible. Therefore, it has been tried to control a pixel by a driver circuit formed over the same substrate as the pixel, which has been conventionally provided outside the substrate.
With the increase of the demand for semiconductor devices, it has been required to manufacture the semiconductor devices at lower temperature in shorter time. A glass substrate, which is superior to a quartz substrate in terms of cost, has been employed as a substrate for a semiconductor device. In the case of forming a TFT with a poly-crystalline semiconductor film over a glass substrate, although a glass substrate is sensitive to heat and easy to deform due to the heat, a semiconductor film can be easily crystallized at low temperature by employing laser annealing.
In addition, compared with another annealing method which uses radiant heat or conductive heat, the laser annealing has advantages that the processing time can be shortened drastically and that a semiconductor film over a substrate can be heated selectively and locally so that almost no thermal damage is given to the substrate.
As laser oscillators used in the laser annealing, there are pulsed laser oscillators and continuous wave (CW) laser oscillators according to their oscillation methods. An excimer laser has advantages of high output power and capability of repetitive irradiation at high repetition frequency. Moreover, a laser beam emitted from the excimer laser has an advantage of high absorption coefficient to a silicon film, which is often used as a semiconductor film. It is preferable to perform the laser irradiation in such a way that a laser beam is transformed into a rectangular, linear, or elliptical shape on an irradiation surface by an optical system and then the beam is scanned relative to the irradiation surface in a minor-axis direction of the rectangular, linear, or elliptical beam, because this method provides high productivity and is superior industrially. At present, a liquid crystal display and an EL (electroluminescent) display are often manufactured by forming TFTs with a semiconductor film crystallized according to this technique.
On the other hand, when a laser beam emitted from a continuous wave laser (the laser beam is hereinafter referred to as a CW laser beam) is transformed into a rectangular, linear, or elliptical shape and the substrate is moved relatively in the minor-axis direction of the rectangular, linear, or elliptical beam, a large grain crystal extending long in the moving direction can be formed. In the case of manufacturing a TFT in accordance with the major-axis direction of the large grain crystal, the TFT has higher mobility than a TFT manufactured with the excimer laser. Since a circuit can be driven at high speed by using the TFT formed with the CW laser beam, a driver circuit for driving a display, a CPU, and the like can be manufactured.
Conventionally, a laser irradiation apparatus shown in FIG. 8 has been known. This laser irradiation apparatus comprises a plurality of cylindrical lens arrays and the like. A laser beam emitted from a laser oscillator 1 is divided into a plurality of beams using a plurality of cylindrical lens arrays 2 to 6 and condensed. Then, after the laser beams are reflected on a mirror 7, the laser beams are condensed into one rectangular, linear, or elliptical laser beam with a doublet cylindrical lens 8 consisting of two cylindrical lenses. After that, the laser beam is delivered vertically to an irradiation surface 9. By delivering the rectangular, linear, or elliptical beam to the irradiation surface while moving the beam relatively in the minor-axis direction of the linear beam, the whole surface of an amorphous semiconductor can be annealed so that the amorphous semiconductor is crystallized, the crystallinity thereof is enhanced, or an impurity element is activated.
However, since the conventional laser irradiation apparatus needs to use a plurality of expensive cylindrical lens arrays and to arrange them so as to form a desired rectangular, linear, or elliptical beam as described above, the apparatus has a problem in that the size and cost thereof increase. Further, since the laser beam, which has been shaped into a rectangular, linear, or elliptical spot is delivered vertically to the irradiation surface, that is, a surface of a semiconductor film formed over a substrate, the beam being incident from above the semiconductor film passes through the substrate and is reflected at a bottom surface of the substrate. Then, the beam incident from above interferes with the beam reflected at the bottom surface. Thus, sometimes a homogeneous crystalline semiconductor film cannot be manufactured.
The present applicant has already suggested a compact and inexpensive laser irradiation apparatus which has overcome the problems of the conventional laser irradiation apparatus. The laser irradiation apparatus is illustrated in FIG. 9. This laser irradiation apparatus uses a convex lens 13 into which the laser beam is incident obliquely so that the laser beam is extended to form a rectangular, linear, or elliptical beam 14. Then, the extended beam is delivered to an irradiation surface 15 obliquely.
That is to say, this laser irradiation apparatus comprises a laser oscillator 11, a mirror 12, the convex lens 13, and the like. A laser beam emitted from the laser oscillator 11 is reflected on a mirror 12 and incident obliquely into the convex lens 13 so that the laser beam is shaped into the rectangular, linear, or elliptical beam 14. The beam 14 is delivered to the irradiation surface 15. With this structure, the apparatus can be made small, and the adverse effect due to the interference caused by the reflected beam from the bottom surface of the substrate can be prevented (see Reference 1: Japanese Patent Application Laid-Open No. 2003-257885).
However, the above laser irradiation apparatus still has the following problem. Although laser annealing is performed with a CW laser beam, for example with a CW laser beam providing 10 W at 532 nm having a rectangular shape of 300 μm in its major-axis direction and 10 μm in its minor-axis direction, the width of the large grain crystal formed by one scanning is as narrow as approximately 200 μm. For this reason, in order to crystallize the whole surface of the substrate effectively by the laser annealing, the laser beam needs to be scanned back and forth while displacing the laser beam by the width of the large grain crystal formed by one scanning of the beam. At this time, if the intensity distribution of the laser beam in the minor-axis direction is not symmetric along a plane which passes through the center of the minor axis of the beam, which is perpendicular to the substrate, and which is parallel to the major axis of the beam, the crystallization state after the laser annealing may be different between back scanning and forth scanning.
However, when the rectangular, linear, or elliptical beam is delivered obliquely to the irradiation surface and the substrate is moved in the minor-axis direction of the beam, the state of the laser beam delivered to the amorphous semiconductor film is different according to the scanning direction of the laser beam as described later. Thus, homogeneous crystallization is difficult to be performed.